A digital image is typically printed or displayed in the form of a rectangular array of “picture elements,” more commonly known as pixels. Digital images are typically represented in a computer by one or more arrays of binary numbers. For example, a monochrome digital image may be represented in a computer by a single array of binary numbers associated with the pixels. Each binary number in the array defines a gray-level value for an associated pixel. The position of the binary number in the array describes the spatial location of the pixel.
A color digital image may be represented in a computer by a plurality (typically three) of arrays of binary numbers. The terms “array” and “color plane” may be used synonymously herein. Each array may represent an axis of a suitable color coordinate system in accordance with trichromatic theory as known to one of ordinary skill in the art. The color of a pixel in a digital image may be defined by an associated binary number from each array. There are a number of color coordinate systems known to one of ordinary skill in the art, for example, red-green-blue (RGB) color coordinate system, cyan-magenta-yellow (CMY) color coordinate system and luminescence (Y), red-yellow (Cr) and blue-yellow (Cb) color coordinate systems.
Large amounts of data are required to represent large, uncompressed color images. Large amounts of data require significant amounts of memory storage in a computer system. While the cost of memory in computer systems has traditionally decreased over time, there are always significant costs associated with memory for computer systems. Additionally, the more data required for a color image, the more difficult it is to transmit the data from place to place in the computer system in a timely manner, thus, requiring faster processors and memory systems. For these reasons, digital image compression techniques have been developed to reduce the costs associated with storing digital color images.
Generally, image compression techniques can be divided into two classes: lossless encoding and lossy encoding. Lossless image compression refers to image encoding and decoding techniques that result in a reconstructed digital image that is identical, pixel for pixel, to the original digital image. Lossy image compression results in a reconstructed digital image that is degraded relative to the original digital image, but may allow for higher compression ratios than those of lossless techniques. One popular type of lossy image compression may be referred to as “transform coding.” Among the most well-known transform coding schemes are those that have been standardized by the Joint Photographic Experts Group (JPEG). JPEG images are widely found in use today on Internet web pages. The term “codec” is an acronym for compressor/decompressor. A codec is any technology for compressing and decompressing data. Codecs may be implemented in software, hardware, or a combination of both. JPEG is a codec.
A conventional multi-channel color compression image processor may include three planes of data. The first plane includes foreground colors, which is usually encoded with a lossless image compression technique to maintain detail. The second plane includes background colors and typically has less detail and is suitable for lossy image compression techniques. The third plane is a binary plane that serves as a switch command between the foreground and background color planes. Multi-channel color compression was developed to produce higher compression ratios by separating information by level of detail, i.e., foreground and background planes. However, in conventional multi-channel color compression, all three planes are maintained at the highest resolution desired, i.e., equal to the targeted marking device. The high resolution may limit compression capability.
Thus, there exists a need in the art for a multi-channel color compression image processing technique that maintains high resolution for edge information but allows reduced resolution for both foreground and background color planes prior to compression and resolution synthesizing after decompression.